JPH04309912A - Image pickup device with optical low-pass filter - Google Patents

Abstract

PURPOSE:To obtain the image pickup device which is simplified in lens constitution and has the optical low-pass filter by obtaining both low-pass effect and aberration correction effect by properly setting the constitution of the optical low-pass filter when the optical low-pass filter is arranged at an optional position of a photographic lens. CONSTITUTION:The image pickup device has the photographic lens and the optical low-pass filter 1 which is arranged at the optional position on the optical axis of the photographic lens and limits picture information of a specific spatial frequency component; and one surface of the optical low-pass filter 1 is in diffraction grating structure with the low-pass effect and the other surface is in an aspherical surface shape.

Description

Detailed Description of the Invention [0001] The present invention relates to an imaging device having an optical low-pass filter, and is used to capture images, such as a video camera or an electronic still camera using a color imaging element. 1
The present invention relates to an imaging device having an optical low-pass filter suitable for dimensional and discrete processing. [0002] Conventionally, in a photographing lens of a video camera or the like that uses an image sensor such as a CCD to obtain an output image by discretely sampling image information, when photographing a subject, the image pickup If an object contains spatial frequency components that exceed the limit frequency of the element, phenomena such as moire fringes that were not originally present in the object or so-called beat, where fine stripes appear as thick stripes with undulations in density, may occur. It will occur. That is, frequency components that cannot be collected by photographing equipment cannot be reproduced as image information, resulting in a phenomenon called waveform distortion (aliasing). In order to suppress this aliasing, an optical low-pass filter is placed in the optical path of the photographing lens of a video camera, etc., and the light flux from the subject is passed through the optical low-pass filter. The light beam is separated into a plurality of directions, and as a result, one point image formed on the imaging surface is separated into a plurality of point images. This limits the high frequency characteristics of the subject and suppresses the effects of aliasing. Conventional optical low-pass filters include those that utilize the birefringence of a uniaxial crystal such as quartz, or those that utilize the diffraction effect of a diffraction grating placed in the optical path of a photographic lens. etc. are used in various ways. Especially in the case of an optical low-pass filter using a diffraction grating, it can be easily made by molding plastic, so it can be obtained at a very low cost.
For this reason, it is widely used in the photographic lenses of recent video cameras and the like. [0007] Optical low-pass filters using diffraction gratings have been proposed, for example, in Japanese Patent Application Laid-Open No. 119063/1982 and Japanese Patent Application Laid-open No. 287921/1982. [Problems to be Solved by the Invention] On the other hand, recent imaging devices for consumer video cameras have optical low-pass effects and high optical performance, and as video cameras have become smaller as a whole, There is a demand for a photographic lens that is designed to be more compact as a whole. In general, it is effective to use an aspherical lens to reduce the number of lenses and downsize the entire lens system while maintaining good optical performance of the photographic lens. For example, Japanese Patent Application Laid-Open No. 2-39011 proposes a compact photographic lens using an aspherical lens. Although it is relatively easy to manufacture an aspherical lens using a plastic mold, there is a problem in that its optical characteristics change greatly due to environmental changes such as temperature and humidity. On the other hand, if an aspherical lens is manufactured using a glass mold, fluctuations in optical properties due to environmental changes such as temperature and humidity will be reduced, but the glass material to be used will be limited and it will be difficult to maintain good surface precision. There is a problem in that it becomes difficult to manufacture. [0012] The present invention makes it possible to easily obtain a predetermined low-pass effect by appropriately setting the configuration of an optical low-pass filter, and to reduce the number of lenses by using an aspheric surface, while improving the environment in which high optical performance is obtained. An object of the present invention is to provide an imaging device having an optical low-pass filter that can always maintain high optical performance with little variation in optical characteristics due to changes. Means for Solving the Problems An imaging device having an optical low-pass filter of the present invention includes a photographing lens and an image of a predetermined spatial frequency component placed at an arbitrary position on the optical axis of the photographing lens. The optical low-pass filter is characterized in that one surface is made of a diffraction grating structure having a low-pass effect, and the other surface is made of an aspherical shape. In particular, the present invention is characterized in that the aspherical shape has a refractive power of approximately 0 paraxially, and the optical low-pass filter is disposed near the aperture stop of the photographing lens. Embodiment FIG. 1 is a schematic diagram of the main parts of an optical system according to Embodiment 1 of the present invention. In this embodiment, a so-called 5-group zoom lens consisting of 5 lens groups as a whole is used as a photographing lens.
An optical low-pass filter is placed near the aperture stop of the photographic lens to obtain a predetermined low-pass effect. In the figure, numeral 3 denotes a first group with positive refractive power that has a focusing function, and numeral 4 denotes a second group with negative refractive power that has a variable magnification function.
The group 5 is a third group that corrects image plane fluctuations due to zooming, 6 is a fourth group that emits the light beam from the third group 5 as a substantially afocal light beam, 1 is an optical low-pass filter, and 2 is an optical low-pass filter. It is an aperture stop and controls the amount of light passing through it. 7 is a fifth group for imaging. The photographic lens of this embodiment changes the magnification from the wide-angle end to the telephoto end by moving the second group 4 and the third group 5 as shown by the arrows. An optical low-pass filter 1 is placed near an aperture stop 2. As shown in FIG. 2, the optical low-pass filter 1 of this embodiment has one surface 1a having a triangular, sawtooth, or trapezoidal cross-sectional shape or a phase-type diffraction grating that exhibits a low-pass effect. The other surface 1b is an aspheric surface for good aberration correction and high optical performance. Further, the aspherical shape at this time has a refractive power of approximately 0 in the paraxial direction. [0020] Here, the refractive power of approximately 0 refers to a lens having a focal length greater than 50 f, where f is the focal length of the entire system. As described above, in this embodiment, one surface of the optical low-pass filter is used as a diffraction grating to obtain a low-pass effect, and the other surface is provided with an aspherical surface, thereby reducing the number of lenses when constructing the photographic lens, especially the optical resolution. The number of lenses in the fifth lens group for images is, for example, about 3 to 5, so that various aberrations such as spherical aberration and coma can be corrected well. [0022] Also, when an optical low-pass filter is molded using a plastic mold with the aspherical surface having a paraxial refractive power of approximately 0, focus shift due to environmental changes such as temperature and humidity, etc. This reduces fluctuations in optical properties. FIG. 5 is a cross-sectional view of a lens in Numerical Example 1, which will be described later, using the zoom type shown in FIG. FIG. 3 is an explanatory diagram showing the principle of image separation of the optical low-pass filter according to the present invention. In the figure, 8 is an optical low-pass filter, and 9 is an imaging surface. The image separation width on the imaging surface 9 by the optical low-pass filter 8 is δ, the prism angle and refractive index of the optical low-pass filter 8 are θ and n, respectively, and the distance between the optical low-pass filter 8 and the imaging surface 9 is When L is the image separation width δ, the image separation width δ is expressed by the following equation. δ=2Ltan(n-1)θ ‥‥‥‥(1
) However, for the sake of simplicity, the fifth group 7 is omitted in the figure. In order to remove false color signals from a reproduced image obtained from a single-plate color image sensor, the image separation width δ of the optical low-pass filter 8 is suitably approximately 1/2 the pitch of the color separation stripe filter. However, when an optical low-pass filter is placed inside a zoom lens, it must be placed in a position that does not interfere with the movement of the lens, so the apparent position of the optical low-pass filter relative to the imaging surface changes due to zooming. I will do it. That is, in zooming, even if the image separation width δ is appropriate when the zoom lens is at the wide-angle end, the image separation width δ may be inappropriate when it is at the telephoto end. Therefore, the optical low-pass filter 8
need to be placed. In this embodiment, the lens is disposed near the aperture stop on the image plane side of the magnification changing section, so that the image separation width due to the low-pass effect remains constant even when the magnification is changed. FIG. 4 is a schematic diagram of the main parts of an optical system according to a second embodiment of the present invention. In this example, a zoom lens consisting of four lens groups as a whole is used as the photographic lens, and an optical low-pass filter similar to that in Example 1 of FIG. 1 is placed near the aperture stop of the photographic lens. Similar to No. 1, a predetermined low-pass effect and aberration correction effect are obtained. In FIG. 4, 43 is a first group having a fixed positive refractive power, 44 is a second group having a negative refractive power and has a variable power function;
is an optical low-pass filter having the same shape as in the first embodiment. 2 is an aperture stop, 45 is a third group having a fixed positive refractive power, and 46 is a fourth group having a focusing function and correcting image plane fluctuations due to zooming. In this embodiment, the second lens group 44 and the fourth lens group 46 are moved as shown by the arrows to change the magnification from the wide-angle end to the telephoto end. Focusing is performed by moving the fourth group 46 on the optical axis. In this embodiment as well, as in the first embodiment shown in FIG. 1, the number of lenses is reduced by providing an aspherical surface on the other surface of the optical low-pass filter, and in particular, the number of lenses in the third group 45 is reduced. The entire lens system is simplified. In this embodiment, the optical low-pass filter is placed closer to the object than the fourth group that moves during zooming. Therefore, the optical distance between the optical low-pass filter and the imaging surface changes due to zooming, and the low-pass effect changes. Therefore, in this embodiment, various constants such as the refractive power, principal point spacing, and movement conditions of each lens group are appropriately set so that changes in the image separation width on the imaging surface do not pose a practical problem. I'm keeping it small. FIG. 6 is a cross-sectional view of a lens according to Numerical Example 3, which will be described later, using the zoom lens shown in FIG. Next, numerical examples of the present invention will be shown. In the numerical examples, Ri is the radius of curvature of the i-th lens surface from the object side, Di is the thickness and air gap of the i-th lens from the object side, and Ni and νi are the curvature radius of the i-th lens from the object side, respectively. These are the refractive index and Abbe number of glass. The aspherical shape has an X axis in the optical axis direction, an H axis in a direction perpendicular to the optical axis, and a paraxial radius of curvature R with the direction of light traveling as positive.
When A, B, C, D, and E are each aspherical coefficients, 00
39] [Equation 1] Numerical Example 1 F= 1~7.6 FNO=
1:2.04~2.25 2ω=48.0°
~ 6.6° R 1 = 6.898
D1=0.166 N1=1.80
518 ν 1= 25.4 R 2=
3.480 D2= 0.833
N2=1.51633 ν2=6
4.1 R3=-13.260D
3=0.027
R4=
3.156 D4= 0.500
N 3 = 1.60311 ν 3 = 60.
7 R 5 = 13.219 D 5
= variable
R6=5.2
79 D 6 = 0.111 N 4
=1.77250 ν 4= 49.6
R7=1.232 D7=0
．． 381
R8=-1.710
D8=0.111 N5=1
．． 77250 ν 5= 49.6 R
9= 1.325 D 9= 0.3
19 N 6=1.84666 ν 6
= 23.8 R10=4306.543
D10= variable
R11=
−2.083 D11= 0.111
N7=1.77250 ν7=49
．． 6 R12=-5.425 D1
2 = variable
R13=2.
071 D13= 0.583 N
8=1.51633 ν 8=64.1
R14= -2.284 D14=
0.208
R15= Aspherical surface
D15=0.138 N9=1.4
9171 ν 9= 57.4 R16
= Lattice plane D16= 0.138

R17= Aperture D17=
0.28
R18=1.
057 D18= 0.458 N1
0=1.48749 ν10=70.2
R19= 4.070 D19=
0.104
R20=5.02
3 D20= 0.111 N11=
1.76182 ν11= 26.5
R21=0.919 D21=0.
804
R22=3.354
D22=0.291 N12=1.
60311 ν12= 60.7 R2
3= -2.018 [Table 1] 15th aspherical surface R0 = ∞ B = -7
．． 896×10−2C = −2.886×10−2
D = −7.533×10−3 Numerical example
2 F= 1~7.6 FNO=
1:2.04~2.25 2ω=48.0°
~ 6.6° R 1 = 7.843
D1=0.166 N1=1.80
518 ν 1= 25.4 R 2=
3.578 D2= 0.833
N2=1.51633 ν2=6
4.1 R3=-14.766 D
3=0.027
R4=
3.392 D4= 0.500
N3=1.69680 ν3=55.
5 R 5 = 15.466 D 5
= variable
R6=5.0
28 D 6 = 0.111 N 4
=1.77250 ν 4= 49.6
R7=1.225 D7=0
．． 381
R8=-1.696
D8=0.111 N5=1
．． 77250 ν 5= 49.6 R
9= 1.308 D 9= 0.3
19 N 6=1.84666 ν 6
= 23.8 R10=1009.983
D10= variable
R11=
-2.084 D11= 0.111
N7=1.77250 ν7=49
．． 6 R12=-5.427 D1
2 = variable
R13=2.
060 D13= 0.583 N
8=1.51633 ν 8=64.1
R14= -2.298 D14=
0.208
R15= Aspherical surface
D15=0.138 N9=1.4
9171 ν 9= 57.4 R16
= Lattice plane D16= 0.138

R17= Aperture D17=
0.28
R18=1.
078 D18= 0.458 N1
0=1.51633 ν10=64.1
R19= 3.759 D19=
0.129
R20=4.80
7 D20= 0.111 N11=
1.80518 ν11= 25.4
R21=0.924 D21=0.
762
R22=3.096
D22=0.291 N12=1.
60311 ν12= 60.7 R2
3= -2.024 [Table 2] 15th aspherical surface R0 = ∞ B = -7
．． 815×10−2C = −2.924×10−2
D = −6.632×10−3 Numerical example
3 F= 1~5.75 FNO=
1:2.05~2.61 2ω=50.4°
~9.4° R1=9.759
D1=0.132 N1=1.80
518 ν 1= 25.4 R 2=
2.884 D2= 0.500
N2=1.60311 ν2=6
0.7 R3=-9.827D
3=0.029
R4=
2.312 D4= 0.323
N 3 = 1.71999 ν 3 = 50.
3 R 5 = 7.262 D 5
= variable
R6=19.6
40 D 6 = 0.088 N 4
=1.78590 ν 4= 44.2
R7=0.820 D7=0
．． 347
R8=-1.194
D 8 = 0.088 N 5 = 1
．． 51823 ν 5= 59.0 R
9= 1.194 D 9= 0.2
64 N 6=1.80518 ν 6
= 25.4 R10 = -75.158
D10= variable
R11=
Lattice plane D11= 0.147 N
7=1.49171 ν 7=57.4
R12=Aspherical surface D12=0.
147
R13= Aperture
D13=0.18
R14
= 2.707 D14 = 0.323
N 8=1.60311 ν 8=
60.7 R15=-5.323
D15= variable
R16=
3.297 D16= 0.102
N9=1.84666 ν9=23.8
R17= 1.304 D17=
0.426 N10=1.51633
ν10= 64.1 R18= -2.4
78 D18 = 0.022

R19= 4.565 D19=
0.220 N11=1.51633
ν11= 64.1 R20= ∞
[Table 3] 12th aspherical surface R0 = ∞ B = 4
．． 701×10-2C = 2.482×10-2
D=-1.341×10-2 [Effects of the Invention] According to the present invention, by appropriately setting the configuration of the optical low-pass filter as described above, a predetermined low-pass effect can be easily obtained. To achieve an imaging device having an optical low-pass filter capable of always maintaining high optical performance with little fluctuation in optical characteristics due to environmental changes when obtaining high optical performance while reducing the number of lenses using an aspheric surface. Can be done.

[Brief explanation of drawings]

[Figure 1] Schematic diagram of the main parts of the optical system of Example 1 of the present invention

[
Figure 2: Cross-sectional view of essential parts of the optical low-pass filter in Figure 1

[Figure 3] An explanatory diagram of the optical action of the optical low-pass filter in Figure 1

[Fig. 4] Schematic diagram of the main parts of the optical system of Example 2 of the present invention

[
Fig. 5 Lens sectional view of Numerical Example 1 of the present invention

[Figure 6
] Lens sectional view of numerical example 2 of the present invention

[Figure 7]
Various aberration diagrams at the wide-angle end of Numerical Example 1 of the present invention

[Figure 8]
Intermediate aberration diagrams of Numerical Example 1 of the present invention

[Figure 9]
Various aberration diagrams at the telephoto end of Numerical Example 1 of the present invention

[Figure 10]
Various aberration diagrams at the wide-angle end of Numerical Example 2 of the present invention

[Figure 11]
Intermediate various aberration diagrams of numerical example 2 of the present invention

[Figure 12]
Various aberration diagrams at the telephoto end of Numerical Example 2 of the present invention

[Figure 13
] Various aberration diagrams at the wide-angle end of Numerical Example 3 of the present invention

[Figure 1
4] Intermediate various aberration diagrams of numerical example 3 of the present invention

[Figure 1
5] Various aberration diagrams at the telephoto end of Numerical Example 3 of the present invention

[Explanation of symbols]

1. Optical low-pass filter 2 Aperture diaphragm 3 1st group 4 2nd group 5 3rd group 6 4th group 7 5th group 8 Diffraction grating 9 Imaging surface 43 1st group 44 2nd group 45 3rd group 46 4th group

Claims (5)

[Claims] Claim 1: A photographing lens; an optical low-pass filter disposed at any position on the optical axis of the photographic lens for limiting image information of a predetermined spatial frequency component; An imaging device having an optical low-pass filter, characterized in that one surface is made of a diffraction grating structure having a low-pass effect, and the other surface is made of an aspherical shape. 2. An imaging device having an optical low-pass filter according to claim 1, wherein the aspherical shape has a paraxial refractive power of approximately zero. 3. The imaging device having an optical low-pass filter according to claim 2, wherein the optical low-pass filter is arranged near an aperture stop of the photographing lens. 4. The photographing lens includes, in order from the object side, a first group with a positive refractive power having a focusing function, a second group with a negative refractive power having a variable power function, and a second group with a negative refractive power having a variable power function, and a second group with a negative refractive power having a variable power function, and a second group with a negative refractive power that corrects image plane fluctuations due to variable power. 3rd to do
2. The optical low-pass filter according to claim 1, comprising five lens groups: a lens group, a fourth group that makes the light beam from the third group a substantially afocal light beam, and a fifth group for imaging. An imaging device having: 5. The photographing lens includes, in order from the object side, a first group having a positive refractive power, a second group having a negative refractive power having a variable power function, a third group having a positive refractive power, and an image forming unit that is attached to an image due to the variable power. 2. An imaging device having an optical low-pass filter according to claim 1, further comprising a fourth lens group having a surface variation correction and focusing function.